Question about compression. at DVinfo.net

Steve Witt · October 4th, 2005, 02:49 PM

When you record with a DV camera, and inside the camcorder the information is changed from analog to digital and is then "compressed" before going onto the DVcassette or other storage device, is the compression that just happened here permanent...or is it "decompressed" when played back? Or, is what you see played back exactly the way it was on the tape (with original compression)?

My second question is, when you go to edit in your computer and capture the video from the DVcassette to the harddrive to begin working, is the information the same size as it was on the cassette or was it compressed a little more again when captured to the harddrive?

Thanks.

Chris Hurd · October 4th, 2005, 03:19 PM

DV is compressed in the camera at a 5:1 ratio. What you see on playback is how it was recorded (and compressed). Capturing DV to a computer through FireWire does not change the compression. Hope this helps,

A. J. deLange · October 4th, 2005, 08:46 PM

When DV is recorded the video signal are compressed and the video stored in compressed form written to tape. When you transfer to computer the compressed data is copied to the computer disk. If you were to take a clean DV tape and put it into your camera and play back over fire wire into the camera while recording the new DV tape would (barring dropouts) be the same as the original and, in theory, you could then transfer that tape to disk, copy it again and keep doing this until you got bored or ran out of tapes.

Now you can't look at compressed data on a computer or studio monitor and you can't edit compressed data so when you play back for viewing or editing the compression has to be reversed. If, after editing, you wish to output to another DV tape the processed video must be recompressed. If you wish to output to DVD it must be recompressed and then compressed again. Compression throws away some picture information. If processing is done this results in some further loss so that the recompressed picture will be a little worse than the original. What this means practically is that you shouldn't copy a DV tape to the computer, do gamma correction, recopy to tape and then come back later to do color correction on the copy. Both gamma correction and color correction (and any other filtering) should be done at the same time. This doesn't mean you can't do gamma correction tonight and color correction tomorrow night if you are using software like FCP because all the processing you have specified in editing sessions is redone each time you render using decompressed data taken each time from the original DV files. Final recompression is done upon the renderd files when you export at the completion of editing.

October 4th, 2005, 02:49 PM	#1
Steve Witt Major Player Join Date: Feb 2005 Location: kentucky, USA Posts: 429	Question about compression. When you record with a DV camera, and inside the camcorder the information is changed from analog to digital and is then "compressed" before going onto the DVcassette or other storage device, is the compression that just happened here permanent...or is it "decompressed" when played back? Or, is what you see played back exactly the way it was on the tape (with original compression)? My second question is, when you go to edit in your computer and capture the video from the DVcassette to the harddrive to begin working, is the information the same size as it was on the cassette or was it compressed a little more again when captured to the harddrive? Thanks.

October 4th, 2005, 03:19 PM	#2
Chris Hurd Obstreperous Rex Join Date: Jan 2001 Location: San Marcos, TX Posts: 27,368 Images: 513	DV is compressed in the camera at a 5:1 ratio. What you see on playback is how it was recorded (and compressed). Capturing DV to a computer through FireWire does not change the compression. Hope this helps, __________________ CH Search DV Info Net \| 20 years of DVi \| ...Tuesday is Soylent Green Day!

October 4th, 2005, 08:46 PM	#3
A. J. deLange Major Player Join Date: Jun 2004 Location: McLean, VA United States Posts: 749	When DV is recorded the video signal are compressed and the video stored in compressed form written to tape. When you transfer to computer the compressed data is copied to the computer disk. If you were to take a clean DV tape and put it into your camera and play back over fire wire into the camera while recording the new DV tape would (barring dropouts) be the same as the original and, in theory, you could then transfer that tape to disk, copy it again and keep doing this until you got bored or ran out of tapes. Now you can't look at compressed data on a computer or studio monitor and you can't edit compressed data so when you play back for viewing or editing the compression has to be reversed. If, after editing, you wish to output to another DV tape the processed video must be recompressed. If you wish to output to DVD it must be recompressed and then compressed again. Compression throws away some picture information. If processing is done this results in some further loss so that the recompressed picture will be a little worse than the original. What this means practically is that you shouldn't copy a DV tape to the computer, do gamma correction, recopy to tape and then come back later to do color correction on the copy. Both gamma correction and color correction (and any other filtering) should be done at the same time. This doesn't mean you can't do gamma correction tonight and color correction tomorrow night if you are using software like FCP because all the processing you have specified in editing sessions is redone each time you render using decompressed data taken each time from the original DV files. Final recompression is done upon the renderd files when you export at the completion of editing.